1. Disopyramide (8 X 10(-5)-4 X 10(-4) M) contracted the rabbit aortic strips and this could be prevented by verapamil or by omitting Ca2+ from Krebs solution. 2. Disopyramide (1.5-6 X 10(-5) M) significantly potentiated the contractile response of the rabbit aortic strips to noradrenaline, clonidine and methoxamine but not that of potassium chloride. 3. Disopyramide (2-6 X 10(-5) M) attenuated the contractile response of the rabbit pulmonary artery to transmural electrical stimulation but potentiated response to noradrenaline. Similar results were observed with the portal vein. 4. The relaxant effect of acetylcholine, on the rabbit aortic ring precontracted with noradrenaline, was blocked by disopyramide while the relaxant effect of adenosine-5'-triphosphate (ATP) was not blocked.